The present invention relates to the control of dissolved gases in liquids including beverages and particularly beer and to the dispense of beverages under pressure at a tap or other dispense device. The most common gases to be controlled in the beverages are carbon dioxide and nitrogen. The raising or lowering of a concentration of a gas takes place in a contactor module containing hollow fiber membranes wherein the beverage flows through the shell side or the bore side of the fibers in the module and gas is controlled by partial pressure regulation on the other side of the hollow fibers in the module.
Carbonation of liquids, particularly for beverages, has taken place for many years. Control of the degree of dissolution of carbon dioxide and other gases in liquids has led to a great deal of experimentation. In some instances, nitrogen has been used in the production and packaging of beers and other beverages primarily to exclude oxygen from the feed water and from contact with the final brewed or bottled product. In addition, it has been found desirable to use nitrogen in a dissolved state in alcoholic beverages, particularly beers, so as to influence the presentation of the beer when the beer is dispensed from a tap into the glass or mug.
Depending on the type of beer the carbonation varies, for instance, for a lager beer generally the carbonation level is above about 2.0 volumes of carbon dioxide per volume of liquid, and for the dark stout beers that level is about 1.0. Many customers, particularly in Europe, express a preference for a tight long-lasting head on dispensed beer. In spite of the presence of various long chain molecules in beers, which molecules have surfactant properties, the desired presentation of a tight long lasting head cannot be achieved with only carbon dioxide in solution. This is true because carbon dioxide is able to permeate rapidly through the thin walls of the initially formed bubbles on the surface of a dispensed beer and hence is lost to the atmosphere which contains a low concentration of carbon dioxide.
It would seem that because the carbon dioxide is supersaturated in the beer that the potential reserve of additional carbon dioxide to replace lost gas would be available. However, this is not normally true because the beer is cold and because modern glass washing methods do not create surface scratches and/or leave deposits which will nucleate carbon dioxide from solution after the beer has come to rest in the glass.
It is known that dissolving a quantity of a weakly soluble gas, conventionally nitrogen, in beer prior to dispense provides high quality presentation in the form of a stable white foam head. Because of its low solubility nitrogen gas which has been pre-dissolved in beer at elevated pressure will very rapidly precipitate out of solution when the beer drink flows through the dispense tap. This precipitation is in the form of a very fine dispersion of small bubbles which approaches its new lower equilibrium concentration at atmospheric pressure when the beer is dispensed.
Because these initially formed nitrogen bubbles are very small, they float slowly to the surface of the beer and some nucleate precipitation of dissolved carbon dioxide gas which enters them, causing them to grow and float faster. The small bubbles which collect at the surface thus contain nitrogen and a mixture of carbon dioxide and nitrogen gases. Because nitrogen, in comparison to carbon dioxide, is less able to permeate through the bubble wall, these bubbles are relatively stable, although they are losing carbon dioxide by permeation to the atmosphere. That loss tends to be made up by further carbon dioxide arising from the bulk of the beer in the glass. Hence the "head" on a nitrogenated beer lasts longer and is more appealing to most customers.
At pubs and restaurants, most beers are transferred by means of pressure displacement, often supplied by carbon dioxide creating a high pressure of carbon dioxide in the keg. Fast displacement of beer by use of high carbon dioxide pressure, provides the risk of over carbonation of the beer. Over carbonation can lead to break out of carbon dioxide in the tubing upstream of the dispense tap when dispensing from a keg to a tap if there is a significant pressure drop in the delivery tubing. This leads to beer loss through "fobbing" i.e., production of excess foam before dispense and at the tap. In an attempt to prevent over carbonation a mixture of nitrogen and carbon dioxide gases has been used for pressure dispense of kegged beers. Although this technique helps to lessen the likelihood of over carbonation, control of a precise amount of carbonation is not feasible by this means.
It has been claimed that there is a causal relationship between the use of nitrogen in production and mixed gas in dispense. The reasoning is that if a beer has been nitrogenated initially then it should be dispensed with a mixed gas in order to maintain that nitrogenation to achieve the desired presentation effects. However, there are three implied requirements which are not independently achievable with the mixed gas dispense principle. These requirements are (1) a maximum total head pressure on the keg in order to achieve fast dispense flow rates; (2) the correct partial pressure of carbon dioxide to avoid over carbonation; and (3) the correct nitrogen partial pressure to maintain nitrogenation. No significant amount of nitrogenation of a keg beer will take place from the mixed gas pressure used for transport because at best only an equilibrium of partial pressures will be established and diffusion mobility of dissolved gases is very low in stagnant liquid layers. However, nitrogen can be lost to the head space from an initially nitrogenated beer. Commercial factors dictate in practice that the two most important requirements are a maximum total head pressure on the keg and the correct partial pressure of carbon dioxide. As a result, dispense with mixed gas is always tailored to maintaining beer carbonation and maximizing speed of dispense as opposed to maintaining the correct nitrogen content for the appeal in presentation.
U.S. Pat. No. 5,565,149 provides a process to nitrogenate beer and/or control the carbon dioxide content of the beer. In this patent, certain membrane modules are used to control the dissolution of carbon dioxide and nitrogen in beer and other liquids or beverages. While the gas dissolution is adequately controlled, the speed at which the beer can be dispensed repeatedly while achieving and maintaining the level of nitrogen and carbon dioxide in the beer is not sufficient in certain circumstances. In the reference, it is noted that when drawing the beer from a tap it is necessary to allow at least 40 seconds for the nitrogen and carbon dioxide levels to be reached for the next draw. In a busy tavern, pub or restaurant, it is frequently necessary to draw beer from the tap every 8 or 10 seconds.
The present invention provides a process and apparatus to dissolve gases such as carbon dioxide and/or nitrogen in beer and the like. The present invention will (1) provide the correct partial pressure of carbon dioxide to avoid either high or low carbonation; (2) provide the correct- partial pressure of nitrogen in the beer for a high quality presentation to the customer; and (3) permit rapid draw from a tap as frequently as every 8 or 10 seconds while providing and maintaining the desired dissolved gas content in the drawn beer.